1. Field of the Invention
The present invention relates to a structure of a semiconductor device having an active area on the heterojunction of a nitride semiconductor.
2. Description of the Related Art
As a semiconductor device using a compound semiconductor, particularly, a high output and high frequency device, an HEMT (High Electron Mobility Transistor) using GaN can be taken as an example. A schematic cross-sectional structure of an HEMT device 90 is illustrated in FIG. 10. In FIG. 10, a channel layer 93 and an electron supply layer 94 are formed by epitaxial growth on a substrate 91 with a buffer layer 92 interposed therebetween. The channel layer 93 is formed of semi-insulating (non-doped) GaN, and electron supply layer 94 is formed of n-AlGaN (to be exact, n-type AlxGa1-xN, x is about 0.20). A two-dimensional electron gas layer is formed on the channel layer 93 side of the interface between the channel layer 93 and electron supply layer 94. The two-dimensional electron gas layer is formed between a source electrode 95 and drain electrode 96 to allow current to flow between the source and drain electrodes 95 and 96. ON/OFF of the two-dimensional electron gas channel is controlled by voltage applied to a gate electrode 97, whereby switching operation is performed. At this time, the speed (mobility) of the electron in the two-dimensional electron gas becomes extremely high, thereby allowing high-speed operation. Further, since the GaN has a larger band gap than that of GaAs, etc., the HEMT device 90 exhibits a high dielectric breakdown voltage and can perform high power operation. In order to obtain favorable amplification characteristics or switching characteristics in this configuration, it is necessary to increase on/off ratio of current flowing between the source and drain electrodes 95 and 96 or on/off ratio of a resistance therebetween. Note that FIG. 10 illustrates the simplest structure of the HEMT device, and the actual structure thereof often differs from that of FIG. 10, wherein, for example, the shape of a contact between source electrode 95 and electron supply layer 94, shape of a contact between the drain electrode 96 and electron supply layer 94, and shape around the gate electrode 97 are actually more optimized than illustrated.
The channel layer 93 and electron supply layer 94 are formed by epitaxial growth on the substrate 91, and the characteristics of the HEMT device 90 are significantly influenced by the crystallinities of the channel layer 93 and electron supply layer 94. The crystallinities of the channel layer 93 and electron supply layer 94, and cost of the HEMT depend strongly on the substrate 91, so that the selection of the material of the substrate 91 is an important factor. For example, the substrate 91 may be a sapphire substrate, a semi-insulating SiC substrate, or the like. However, since it is difficult to directly form the channel layer 93 (semi-insulating GaN) having favorable crystallinity on such a material (wafer), the buffer layer 92 made of a material other than above needs to be formed between the channel layer 93 and substrate 91. Further, since the sapphire substrate and semi-insulating SiC substrate are expensive, the use of another wafer has been studied.
Recently, as a GaN wafer, an n-GaN (n-type GaN) wafer of a manageable size can be obtained at low cost for use as the substrate 91. For example, there is disclosed as a fourth embodiment of Patent Document 1, an HEMT device having a structure using a GaN wafer as the substrate 91. In this case, the semi-insulating GaN having favorable characteristics can comparatively easily be formed on the substrate 91 as the channel layer 93 due to the use of the same material.
Further, from the viewpoint of reducing the on-resistance of the HEMT device 90, there is disclosed in, e.g., FIGS. 13 and 14 of Patent Document 2, a technique that uses a penetrating electrode that penetrates the HEMT device from the source electrode 95 to the substrate 91 so as to forcibly make the potentials of the source electrode 95 and substrate 91 equal to each other. According to this technique, a rear surface electrode formed widely on the rear surface of the substrate 91 can be used as the source electrode. Thus, as described in paragraph [0046] of Patent Document 2, a source electrode pad need not be formed on the front (upper) surface side of the HEMT device 90, thereby allowing efficient use of the entire chip.